In the process of recovering hydrocarbons from a subterranean formation through a wellbore penetrating the formation, it is common to stimulate production and facilitate transport of the hydrocarbons to the wellbore by the process of hydraulic fracturing. In the hydraulic fracturing process, a fluid is pumped into the zone of interest at a pressure high enough to overcome the reservoir pressure and pressure transmitted by the overburden to a point where the rock within the formation fractures. In a typical fracture operation, following the initiation of a fracture, additional quantities of fracturing fluid are pumped into the formation to extend and widen the fractures. Later-pumped quantities of fluid typically contain a proppant material, usually in gradually increasing quantities in the fracturing fluid. Upon fracture closure with release of the fracturing fluid pressure the proppant remains within the fracture offering a permeable channel through which formation fluids can flow more easily to the wellbore for production to the surface.
The fracturing fluids, particularly those used to transport proppants generally comprise gelled aqueous fluids having a viscosity sufficient to suspend the proppant within the fluid for pumping and placement within the fractures. Typical gelled fracturing fluids comprise galactomanan polymers which are commonly crosslinked using an organo-metallic compound. Guar and substituted guar polymers such as hydroxypropyl and carboxymethylhydrocypropyl guar are commonly used. For relatively high temperature applications (above about 200.degree. F.) crosslinking of these guar-based polymers is effected with a zirconium, hafnium or titanium crosslinking organo-metallic compound.
At temperatures above about 300.degree.-325.degree. F., however, the stability and, thus, the proppant-carrying capacity of such crosslinked gel fracturing fluids is relatively low. The high temperatures cause a breakdown of the polymer and early release of the proppant from the gel. In the past, in order to reduce the formation temperature effects on a fracturing fluid in high temperature formations, it has been common to pump very large volumes of fluid into a formation as a "pad" without the presence of proppant in order to cool down the formation so that when a proppant-laden fracturing fluid is pumped into the fractures, the temperatures effects of the formation are reduced for a period of time until the formation temperature recovers from the cool-down effects of the initial large pad volume. Such large pad volumes are undesirable because in addition to the high cost of formulating and pumping these large additional volumes, large amounts of polymers are deposited within the fractures after fluid leak-off which polymers can have an adverse affect on fracture conductivity through the proppant pack.